Treatment of gas



Dec. 4, 1934. G. A, BRAGG 1,983,338

TREATMENT OF GAS Filed May 5, 1932 1 INVENTOR. 6/ /b err 5/229 Patented Dec. 4, 1934 TREATMENT OF GAS Gilbert A. Bragg, deceased, late of Pittsburgh, Pa., by Fidelity Trust Company, executor, Pittsburgh, Pa., assignor to The Koppers Gompany of Delaware, Pittsburgh, Pa., a corporation of Delaware Application May 5, 1932, Serial No. 609,462

2 Claims.

This invention relates to the electrical treatment of gas for the removal of suspended material therefrom. It relates especially to the electrical precipitation of pitch and similar suspended 5 material from fuel gases, such as producer gas, coal gas, and the-like, and to the flushing of the collecting electrodes of apparatus employed for this purpose.

Electrical precipitation processes and apparatus have been employed in the past to remove various types of suspended matter from various gases. For example, cement dust, metallic fumes, smoke, and tar, have been removed from smelter and furnace stack gases, air, and fuel gases by electrical precipitation, and there have beenother applications of this method. Various types of apparatus have also been developed as required by the difi'erent conditions encountered.

In all of the usual types of electrical precipitation apparatus there are two electrodes or sets of electrodes which are usually known as discharge electrodes and collecting electrodes, and which in most cases are insulated from each other and kept-at a high potential difference when the apparatus is in operation. The discharge electrodes usually have a relatively small surface and may be provided with points or sharply curved sections, if desired. They are connected to a generator, transformer, rectifier or other generally have more extended surfaces which may be fiat or curved and which may contain apertures, and these electrodes are usually grounded.

Perhaps the best known apparatus for this purpose is the Cottrell type of electrical precipitator, in which each individual precipitating unit consists of a tubular collecting electrode and a concentric discharge electrode which may be a wire, rod, chain, or the like. The present invention is well adapted for use in conjunction with apparatus of this type but is not limited thereto and may also be used with electrical precipitators in which the collecting electrodes are flat plates or have other form, and in which the discharge electrodes have any desired form. As an example of its application, however, the invention will be described with special reference to its application to electrical precipitators of the vertical tube type.

In electrical precipitators the gas to be cleaned passes through tubes or flues whlcliiusually constitute the collecting electrodes, and the gases are there subjected to a high voltage electrical dis- 55 charge from the discharge electrode. This causes migration of solid or liquid particles carried by the gas to thefiue walls or other collecting electrodes present. r

The subsequent behavior of the material deposited on the collecting electrodes depends on source of high voltage. The collecting electrodes its physical characteristics. Most liquids, for example, trickle down the electrodes and collect in sumps provided to receive them at the base of the treater, while dry granular solids, such as dusts, for example, fall from electrodes after ac cumulating in suflicient quantity, or are readily removed by rapping or tilting the electrodes.

Some kinds of precipitate are only removed from the collecting electrodes with great difficulty, however, and'tend to form tenacious deposits which decrease the efiiciency of the apparatus and which cause arcing and other difliculties. This condition is especially prevalent in the removal of pitch, heavy tar og, and similar material fromfuel gases and the like.

As a specific illustration of the problem with which the present invention deals, it has been found that when pitch is precipitated from producer gas which is used for underfiring coke ovens, for diluting coke-oven gas, and for other purposes, it accumulates on the collecting electrodes and is very diflicult to remove.

External heating does not make the pitch sulficiently fluid to flow from the tubes, water flushing is without eifect, and direct steam seems to make the pitch adhere more tenaciously. It cannot be dislodged by rapping, even when deposits of'considerable depth are formed, and a coating having a maximum thickness of about oneeighth inch is suflicient to decrease the efiici'ency of the apparatus by 10% or more.

An object of the present invention is to provide a process of removing deposits of pitch and. other troublesome material from the collecting electrodes of electrical precipitation apparatus.

A second object of the invention is to provide a method of preventing the formation of troublesome deposits of pitch and the like on the electrodes of electrical precipitators.

Another object of the invention is to provide an improved process of removing suspended matter, such as pitch, from gases such as fuel gas, and suitable apparatus therefor.

The invention has for further objects such other operative advantages and results as are found to obtain in the process and apparatus described and claimed herewith.

I It has been found that if the pipes or other collecting electrodes of electrical precipitators are flushed with solvent for the material removed from the gas, which material tends to form deposits which are to be removed or prevented, satisfactory results are in many instances obtained. For example, in the case of pitch deposits, tar, such as coke-oven tar, with or without the addition of water-gas tar or suitable mineral oils, makes a satisfactory and eflicient flushing fluid, as described in the copending application of E. V. Harlow, Serial No. 605,663, filed April 16, 1932.

' through which the gas preferably passes in a downward direction.

A somewhat similar procedure has been followed in flushing'electrodes withwater, but it has been found that this method isespecially useful and efficient when employed to prevent the formation or accumulation of deposits of pitch and the like by injecting tar or other non-aqueous solvent for the suspended material into the gas from which thematerial is precipitated. The invention is not intended to be limited to the removal of pitch depositsg however, as it may also be employed to remove or prevent the formation of other deposits by flushing with tar or some other suitable and preferably non-aqueous liquid.

In the practice of the process, it is desirable to operate the precipitatonwith downward flow of gas in'the pipes or other flues since most of the flushing liquid, such as tar, will then be thrown out near the top of the flue and will draindown over the entire surface. If, on the other hand, the gas passes upwardly through the flues, the

atomized tar, which is often more readily precipitated than-the pitch occurring in the gas, is deposited chiefly on the lower part of the flue walls, and the upper portions thereof are not flushed thoroughly.

A further advantage of down-draft operation is that the larger particles or droplets of tar,

which tend to precipitate from the gas of their own. accord, are then carried into the flues by gravity, while in an up-draft precipitator these larger particles tend to separate from the gas stream before it reaches the flues, and thus take no partinthefl A two-fluid atomizing nozzle is especially suitable for injecting tar or 'the like into the gas.

In one type of two-fluid nozzle, the tar is dis-i charged through a central orifice, usually of circular cross-section, which is surrounded by an annular orifice through which gas at suitable pressure, such as to'jlOO pounds per square 'inch, is supplied for dispersing the tar.- The pressure of this atomizing gas may, of' course, m made higher or lower, as required.

- A one-fluid nozzle to which tar or other solvent is delivered under a high pressure produces a fairly satisfactory mist, but two-fluid nozzles give a very high degree of dispersion which makes possible a practically homogeneous mixture of the mist with the gas.

' Withthis high degree of dispersion, excellent distribution of the flushing medium results and,

therefore, substantially perfect flushing is accomplished with a very small amount of fluid. For

example, in a commercial scale precipitator, two

quarts of tar per hour per pipe have been found satisfactory for 8" pipes. Thisis a. great advantage since a large amount of flushing fluid has a tendency to cause arcing in the pipes before deposition, and also to form streamers from the bottom 0f the pipes, which streamers are drawn towards the discharge electrode and cause violent arcing.

Another advantage of atomizing the tar into the gas rather than employing other methods of introducing itis that the tar is distributed not only to the pipes or flues but also to the inside of the precipitator shell, the electrode supports, and all other parts of the apparatus where suspendedmaterial may deposit, thereby eliminating accumulations of pitch and dust which might cause stoppages of the flow of gas or grounding of the discharge electrode system.

Furthermore, since the atomized tar or other solvent forms a-suspensign which is sufficiently stable to form an approximately homogeneous mixture with the gas, the flushing fluid reaches all parts of the system freached by the pitch or other material suspended in the gas, and reaches then compressed to a suitable pressure. By using high-pressure gas of the kind treated, change in the composition'of the gas during treatment is avoided.

Furthermore, there are other factors favoring this practice. For example, it would obviously be dangerous to employ compressed air for atomizing tar into a combustible gas. Steam produces a satisfactory dispersion or mist of the flushing 1.10

fluid, but has two inherent defects.

First, the heat of the steam-distills the lighter fractions from the tar, making it more-viscous and contaminating the gas with the distillate. This is especially true when water-gas -tar is 11 mixed with coke-oven tar. Second, the steam condenses and causes arcing in the precipitator,

and at the same time the temperature of the 1 gas and its dew-point with respect to water are raised, which, in most instances, is undesirable.

' Now will be described, by way of example, the application of the invention to the-removal of pitch from fuel gas in a vertical tube type of treater; This will be described with reference to the accompanying drawing, in which Figure 1- is a somewhat diagrammatic view, partly in elevation and partly in vertical section, of apparatus suitable for the practice of the improved process of electrical precipitation;

Fig. 2 is a vertical sectionalifliewpnan enlarged scale of part of the apparatus shown'in Fig.i;and

Fig. 3 is a horizontal section along the line III-III in Fig. 2.

Fuel gascontaining pitch or other suspended matter enters an electrical precipitator 1- through a pipe or main 3. It passes downwardly through distributing vanes 4, and continues through tubes or flues 5, where it is subjected to the influence of a high-voltage electrical discharge which causes migration of the suspended material toward the walls of the tubes. The cleaned gas is then discharged through a pipe '7. a

Discharge electrodes 9v which may have the form of wires, rods, chains, or the like, are sup- 1 ported coaxially within each of the tubes 5 from a support or bus bar 10, which in turn is supported by insulators in insulator compartments 12. A suitable source of high voltage; such as a generator, transformer, or rectifier, is connected to the 150 bus bar 10 and the discharge electrodes 9 by means of one or more lead-in wires 14. The precipitator shell and the collecting electrodes 5 connected thereto are grounded as indicated at 15.

A solvent or vehicle for the suspended material precipitated from the gas in the precipitator 1 is stored in a tank 16. When pitch is removed from fuel gases, such as producer gas for example, this solvent is preferably tar, such as watergas tar for example, which is withdrawn by a pump 18 through a pipe 20 from a tank car (not shown) or other means for shipment or storage.

This tar is delivered through pipes 21, 22 and 23 to the tank 16. From time to time it is withdrawn from tank 16 through pipes 25 and 20, and delivered by the pump 18 through pipes 21 and 27 to a mixing tank 29, where it is mixed with coke-oven tar supplied through a pipe 30, or with other desired constituents to form the flushing mixture.

If it is desired to employ coke-oven tar alone for the flushing mixture, the tank 16 and connectlons thereto may be omitted, or coke-oven tar may be supplied thereto. Coke-oven tar is generally a better solvent or vehicle for pitch and the like than water-gas tar, but a mixture of the two tars is suitable for use over a wider range of temperatures and has other advantages, as set forth in the aforementioned Harlow application.

The tar mixture or other solvent in the tank 29 is heated by a steam coil 31 or other heating means if necessary. This solvent is withdrawn from the tank 29 continuously or intermittently by a pump 33 and delivered through a pipe 34 to a distributing pipe or header 35 from which it enters one or more atomizing nozzles or sprays 37. The pipe 34 is preferably provided with a screen or other means for removing coal dust or other solid particles which might cause stoppage of the sprays.

Any desired number, .type and arrangement of atomizing nozzles may be employed. In the preferred instance, however, a suitable number (four, for example) of two-fluid atomizing nozzles 37 are connected to the header 35 and arranged symmetrically in the tapered upper part of the precipitator chamber. These nozzles are suitably located just below the concentric distributing vanes 4 for equalizing the gas flow, although other locationsgmay sometimes be preferable. Auxiliary flushing sprays 38 may also be connected to the header 35, for the purpose described hereinbelow.

An atomizing fluid or medium, which, in the preferred instance, is cleaned gas discharged from the precipitator 1, is supplied to the nozzles 37 from a pipe 40. Such gas may be supplied from the precipitator outlet pipe 7 to a compressor 42 through a pipe 43. After compression to the desired atomizing pressure, such as 35 to 100 lbs. per square inch, it enters a storage tank or holder 45, from which it is discharged through pipe 40' as required.

The tar mixture or other solvent atomized into the gas by the nozzles 3'7 continues downwardly through the precipitator as a substantially homogeneous mixture with the gas and is precipitated on the collecting electrodes 5 and all other parts of the apparatus on which pitch or other suspended matter contained in the gas is precipitated. This tar forms a flowing protective film on the surfaces of the collecting electrodes, preventing the deposition of pitch on the bare electrode surfaces and dissolving deposits previously formed there.

The tar or other solvent carrying the precipitate from the gas, such as pitch in suspension or solution, drains from the collecting electrodes into 1 pump 18 through pipe 52 and discharged through pipes 21, 22 and 23 to the storage tank 16, or through pipes 21 and 27 to the mixing tank 29 for recirculation, or otherwise disposed of, as by utilizing it for fuel. I

When it is desired to completely remove tar and pitch from the interior of the precipitator, as for inspection or repair, a solvent for the tar, such as solvent naphtha, for example, may be employed to flush out the precipitator, preferably' after discontinuing its operation. This solvent may be withdrawn from a tank 54 through pipes 55 and 20 to the pump 18 which delivers it through pipes 21 and 22 to the header 35 from' which it is discharged through sprays connected thereto, such as sprays 37 or preferably the'special or auxiliary sprays 38.

In a preferred arrangement shown in Fig. 3,

the atomizing sprays 37 are connected to 'the' header 35 which is supplied from pipe 34, and a separate header 58 supplied from pipe 22 is pro-; vided for the sprays 38 the headers 35 and 58 may be interconnected, if desired, or one of them may be omitted, or the special sprays 38 may be omitted In any case, the solvent naphtha or other solvent for tar passes downwardly through the precipitator, completely removing tar and pitch from its interior. It is discharged through pipe 48 to the sump 50, contaminated with tar. From this sump, it may be withdrawn by the pump 18 and returned through pipes 21 and 59 to the storage tank 54, with or without distillation or other purification, or it may be otherwise disposed of.

It will be obvious to those skilled in the art that certain modifications can be made in the several parts of my apparatus and in the several steps of my process, in addition to those described by way of example hereinabove, without departing from the spirit of the invention, and it is the intention that the claims shall cover such modifications as are included within the scope thereof.

What is claimed is:---

1. The process of removing suspended particles from a gas, which comprises atomizing tar into the gas, subjecting the gas and atomized tar to the action of an electric field whereby tar and suspended particles are precipitated, withdrawing substantially clean gas, compressing part of the withdrawn clean gas, and employing clean compressed gas toatomize a further quantity of tar into a further quantity of gas.

2. The process of removing suspended particles from a gas, which comprises atomizing tar into the gas, subjecting the gas and atomized tar to the action of an electric field whereby tar and suspended particles are precipitated from the gas, withdrawing substantially clean gas, separately withdrawing precipitated tar, compressing part of the withdrawn clean gas, and employing the clean compressed gas to'atomize the precipitated tar into a further quantity of gas.

FIDELITY TRUST COMPANY, Executor of the Estate of Gilbert A. Bragg, De-

ceased,

By ALEXANDER P. REED,

Vice-President. 

